Long Term Evolution (LTE) is a radio access technology standardized by the 3rd Generation Partnership Project (3GPP). LTE is based on orthogonal frequency division multiplexing, OFDM, in the downlink and single-carrier frequency domain multiple access, SC-FDMA, in the uplink.
In the time domain in LTE, one subframe of 1 ms duration is divided into 12 or 14 OFDM (or SC-FDMA) symbols, depending on the configuration. One OFDM (or SC-FDMA) symbol consists of a number of sub-carriers in the frequency domain, depending on the channel bandwidth and configuration. One OFDM (or SC-FDMA) symbol on one sub-carrier is referred to as a resource element, RE.
In LTE, no dedicated data channels are used. Instead, shared channel resources are used in both downlink and uplink. These shared resources, which are referred to as DL-SCH and UL-SCH, are each controlled by a scheduler that assigns different parts of the downlink and uplink shared channels to different communication devices (e.g., user equipments) for reception and transmission respectively.
The assignment information for the DL-SCH and the UL-SCH are transmitted in a control region covering n OFDM symbols in the beginning of each downlink subframe. The variable n generally has a value between 1 and 3, inclusively, for all subframe bandwidths except for a bandwidth of 1.4 MHz, in which case n generally has a value between 2 and 4, inclusively. The DL-SCH is transmitted in a data region (e.g., a shared data region) covering the rest of the OFDM symbols in each downlink subframe. The size of the control region (i.e., the value of n) is set per subframe. The size is signalled to the UE for each subframe, as a control format indicator (CFI) value, on a physical control format indicator channel (PCFICH) in the control region. The PCFICH occupies a certain minor, pre-determined part of the control region, thereby making it independent of the number of OFDM symbols currently used for the control region. A CFI value of 1, for instance, indicates that the control region has a size of 1 symbol.
Each assignment for DL-SCH or UL-SCH is transmitted on a physical downlink control channel (PDCCH) located in the control region. There are typically multiple PDCCHs in each subframe, and each UE is required to monitor the PDCCHs to detect the assignments directed to it. A PDCCH is mapped to a number of control channel elements (CCEs), which may be a set of 9 resource element groups (REGs), where each REG may be a group of 4 consecutive REs. A PDCCH consists of an aggregation of 1, 2, 4 or 8 CCEs. These four different alternatives are herein referred to aggregation level 1, 2, 4, and 8, respectively. Each CCE may only be utilized on one aggregation level at the time. The variable size achieved by the different aggregation levels is used to adapt the coding rate to the required block error rate (BLER) level for each PDCCH. The total number of available CCEs in a subframe will vary depending on several parameters, some of which are static (bandwidth and number of antennas), some are semi-static (physical HARQ indicator channel (PHICH) size and PHICH duration), and one is dynamic (number of OFDM symbols used for the control region). Each CCE may consist of 36 REs (9 REGs×4 REs/REG). However, in order to achieve time and frequency diversity for the PDCCHs, each CCE and its corresponding REs are spread out, both in time over the OFDM symbols used for PDCCH, and in frequency over the configured bandwidth. This is achieved through a number of operations including interleaving, and cyclic shifts, etc.
The CCH mapping in the control region is also restricted in order to simplify the UE implementation. Depending on a radio network temporary identifier (RNTI), subframe number, and CCE aggregation level, only a limited set of CCEs need to be searched for PDCCHs by a UE. These CCE sets are referred to as common search spaces and UE-specific search spaces depending on if the PDCCH is aimed at a group of UEs or to a specific UE. The UEs recognize PDCCHs aimed at them by the RNTI tag that is attached to each PDCCH.
The PDCCHs are mapped on the control region of the subframe, which can consist of a number of OFDM symbols. The control region size can be varied from one subframe to a subsequent subframe, such as from two OFDM symbols to three OFDM symbols. Increasing the size of the control region may decrease the size of the data region in the subframe, because there may be only a fixed number of OFDM symbols available per subframe (i.e., 12 or 14 OFDM symbols for normal and extended cyclic prefix respectively). This may in turn decrease the available capacity for DL-SCH in the data region. Hence, the control region is an overhead that competes with DL-SCH for resources (i.e., the downlink peak throughput is affected if the control region is larger than what is absolutely required to carry the control channels). If the control region is too small, however, it may not be large enough to carry all PDCCHs required for both DL and UL assignments. This may prevent the sending of all the DL and UL assignments for the UEs, which may result in the DL and/or UL throughput being degraded due to the limited capability to assign DL-SCH and UL-SCH resources to the UEs. Accordingly, it is not optimal to use a fixed control region size for each downlink subframe.
What is desired, therefore, are systems, methods, and apparatuses for selecting, for each downlink subframe, the size of a control region for the downlink subframe.